Advanced Reactor Development and Technology

Heavy Liquid Metal Reactor Development

Argonne has traditionally been the foremost institute in the U.S. for development of technologies for advanced nuclear power systems. The Heavy Liquid Metal Coolant (HLMC) reactor research initiative is one element of efforts at Argonne to address reactor technologies that can fulfill the requirements for nuclear power to be viable as an important energy resource for the 21st century, both for domestic (U.S.) and international utilization. In this effort, reactor concepts are developed and evaluated in relation to proposed specific applications. One such concept is intended to be a low-cost contender for commercial electricity production. The approach is to achieve capital and operating cost savings through extreme measures of simplifying the system; i.e., by providing a robust and passively safe system with minimal maintenance needs, providing a small, modular pool-type configuration that lends itself to the economy of factory fabrication and overland transportation, and providing an extremely long-life core design which eliminates fuel shuffling and partial reloads and requires refueling outages only at very long intervals (≈ 15 years). The reactor system is required to be exportable to developing countries, and so the approach includes measures for proliferation resistance. The system described (SUPERSTAR) is sized for the export market at about 120 MWe; however, plant power can be varied for greater needs by coupling multiple modules. The design approach is focused upon that which offers the possibility to achieve deployment in an early time frame.

SUPERSTAR is the latest in a series of Lead-Cooled Fast Reactor (LFR) Small Modular Reactor (SMR) concepts developed at Argonne since 1997 resulting in the earlier Secure Transportable Autonomous Reactor with Liquid Metal (STAR-LM) and Small Secure Transportable Autonomous Reactor (SSTAR) concepts for international deployment, or deployment on remote or small electrical power grids. Small- (< 300 MWe) and medium (300 to 700 MWe)-sized reactors are better suited to growing economies and infrastructures of developing nations than classical economy-of-scale plants. They do not require as large financing as economy-of-scale plants that could be beyond the reach of affordability for developing nation utilities and can be deployed in increments with each reactor constructed in a shorter time than economy-of-scale plants enabling planning and financing to be spread out over time. They are “right sized” for initially small but fast growing electric grids for which changes in load demand can be significant. Small- and medium-sized reactors provide greater base load flexibility and SUPERSTAR can provide load following capability over a wide range of power. SUPERSTAR can provide energy security for nations not wanting the expense of an indigenous fuel cycle and waste repository infrastructure but willing to accept the guarantee of services from a regional fuel cycle center by virtue of a long (15- to 30-year) refueling interval. The initial fissile inventory is large but the one-time initial fissile loading is less than the lifetime 235U consumption of a Light Water Reactor (LWR) for the same energy delivery. Once loaded, SUPERSTAR is fissile self-sufficient with a conversion ratio, CR ≈ 1.0. SUPERSTAR provides proliferation resistance. Access to fuel is restricted. SUPERSTAR does not incorporate any in-vessel fuel handling equipment. There is no onsite refueling equipment. Refueling equipment is brought onsite only at the time of refueling after which it is removed. The long core lifetime further restricts access to fuel by reducing or completely eliminating the need for refueling. The design restricts the potential for misuse in a breeding mode. The core design has a conversion ratio of unity such that it self-generates as much fissile as is consumed; excess fissile is not created. The fuel form is unattractive in a safeguards sense by virtue of high radioactivity due to incomplete removal of fission products during reprocessing.

SUPERSTAR seeks to achieve a low enough cost to be economically competitive with alternative energy sources (e.g., diesel generators in remote locations). It is readily transported and assembled from transportable modules. SUPERSTAR incorporates autonomous load following and an advanced supercritical carbon dioxide Brayton cycle power converter with a control strategy facilitating autonomous load following by the reactor. It is not necessary for operators to change the reactor power through deliberate motion of control rods. The control rods are mainly for startup and shutdown as well as slow compensation of the burnup reactivity swing over the core lifetime. This supports the goal of making SUPERSTAR simple to operate reducing operating staff requirements and providing high reliability.

SUPERSTAR embodies a high level of passive safety reducing the number of accident initiators and the need for safety systems enabling the size of the exclusion and emergency planning zones to potentially be reduced. Four Direct Reactor Auxiliary Cooling System (DRACS) heat exchangers (HXs) for passive emergency decay heat removal are installed inside of the cold pool of the reactor vessel. Each DRACS loop rejects heat to the atmospheric heat sink; the louvers on the air heat exchangers require electrical power to close such that they passively open upon the loss of electrical power. To always maintain a small natural circulation flow of the DRACS circuit intermediate coolant, the louvers incorporate an orifice that results in a small amount of heat rejection to air and natural circulation of the DRACS circuit intermediate coolant.

SUPERSTAR is a natural circulation reactor which is an example of design simplification. The benefits of natural circulation are: 1) eliminating the capital cost of primary coolant pumps; 2) eliminating loss-of-flow accident initiators due to pump coastdown; 3) eliminating downtime due to failures of mechanical pumps; and 4) eliminating the potential problem of erosion of the pump impeller due to high velocities in the dense heavy liquid metal coolant (HLMC). The SUPERSTAR first-of-a-kind concept utilizes an innovative particulate-based metallic (U-Pu-Zr) fuel that doesn’t require a sodium bond eliminating the need to otherwise incorporate sodium inside of a LFR. The approach in incorporating this particulate-based metallic fuel form is to facilitate near-term deployment of a SUPERSTAR first-of-a-kind demonstrator seeking regulatory approval for the use of the particulate-based metallic fuel by taking advantage of the existing experience and database for sodium-bonded cast metallic fuel together with testing and analyses investigating effects of the particulate-based metallic fuel form.

SUPERSTAR incorporates an intermediate heat transport circuit utilizing Pb intermediate coolant to exclude the carbon dioxide working fluid from inside of the containment. This eliminates the need to include a carbon dioxide line break in the containment design basis. Thus, the containment does not need to have a significant pressure retention capability simplifying the containment structural design and reducing the cost associated with the containment. SUPERSTAR incorporates Pb-to-Pb intermediate heat exchangers (IHXs) inside of the reactor vessel that cool down the primary Pb coolant and transfer heat to the intermediate lead coolant. The IHXs consist of Formed Plate Heat Exchanger (FPHE, Heatric Divison of Meggitt (UK), Ltd.) compact diffusion-bonded heat exchangers that provide a large surface area for interfacial heat transfer and low pressure drops.

To summarize, SUPERSTAR is an improved LFR SMR concept integrating the lessons learned, innovations, and best features from earlier Argonne LFR SMR natural circulation concepts as well as other LFR forced flow concepts.